The pellets will swell, but it is minimal in comparison to the amount of trub that might already be in your fermenter if you didn't strain the wort.
Why are you concerned about how much trub will be created?

Thanks for the replies guys. More curious than concerned. I'm fermenting in a bucket with a spigot on it, no secondary. Not wanting to take the lid off, I'll be removing the airlock and putting the pellets in via funnel through the hole in the lid. The existing trub is just below the spigot, so not too worried about solids carryover. I typically bottle the first quart of so to remove 'most' of the trub then fill the keg.

The paint strainer on a collar is great for boiling, use it for the multiple additions wihout having to clean a bunch of bags.

Either way works. If you take the lid off at this point the risk of oxidation/contamination is just as extremely low if you aren't sloppy - with either method. Again, either way your beer should turn out good.